A spin-valve film comprises thin magnetic layers which are laminated.
Resistance of the spin-valve film is changed by an external magnetic field. This change is known as MR (magnetoresitive) effect. The MR effect is caused by various physical effects. GMR (giant magnetoresistive) effect and TMR (tunneling magnetoresistive) effect are well known.
The spin-valve film is comprised of two or more ferromagnetic layers separated by a spacer layer. The magnetoresistance is determined by the relative orientations of magnetization of adjacent ferromagnetic layers. When two ferromagnetic layers have magnetizations with a parallel alignment the spin-valve film can be in a low resistance state. When the two magnetizations have an anti-parallel alignment the spin-valve film can be in a high resistance state. When angles of magnetizations in the adjacent ferromagnetic layers are at intermediate angles the spin-valve film can be in an intermediate resistance state.
In one of at least two ferromagnetic layers, a magnetic layer whose magnetization is easily variable is known as a free layer. A magnetic layer whose magnetization is not easily variable is known as a reference layer.
By using the magnetoresistance phenomenon, whereby the resistance of the spin-valve film is changed via an external magnetic field, the spin-valve film is used as a magnetic field sensor element. As a result of their good magnetic field sensitivity, the spin-valve film has widely been used in the read-head of HDDs (hard disk drives). Additionally, spin-valve films are used as part of the memory cell of MRAM (magnetic random access memory).
The spin-valve film can detect not only an external magnetic field, but also an external strain. This phenomenon enables us to use the spin-valve film as a strain sensor element or a pressure sensor element. The physical origin for the change in magnetization of a ferromagnetic layer by strain is an inverse-magnetostrictive effect. A brief explanation of this effect as follows.
The magnetostrictive effect is a phenomenon where a magnetic material's shape changes when the magnetization of the magnetic material is changed. The extent of the shape change of the magnetic material is determined by the magnitude of the magnetization and the direction of the magnetization. Therefore, the shape change is controlled by the magnitude of the magnetization and the direction of the magnetization. The amount of change which can occur from the magnetostriction of the magnetic material is described by the magnetostrictive coefficient λ. The magnetostrictive coefficient λ depends on the composition of the magnetic material and the layer structure of the magnetic material.
A phenomenon complementary to the magnetostrictive effect, the inverse-magnetostrictive effect, is also known. With the inverse-magnetostrictive effect, the magnetization of a magnetic material can be changed when the magnetic material is stressed, for example by the application of an external strain. The magnitude of the change in magnetization depends on the magnitude of an external strain and the magnetostrictive coefficient of the magnetic material. The magnetostrictive coefficient relating to the inverse-magnetostrictive effect is same as the magnetostrictive coefficient λ from the magnetostrictive effect because the magnetostrictive effect is converse to the inverse-magnetostrictive effect.
The magnetostrictive effect and the inverse-magnetostrictive effect may have a positive magnetostrictive coefficient or a negative magnetostrictive coefficient. The magnitude and sign of these coefficients depend on the composition of the magnetic material and the layer structure of the magnetic material.
If the magnetic material has a positive magnetostrictive coefficient, the direction of the magnetization of the magnetic material is aligned in the direction of a tensile stress or away from a compressive stress.
On the other hand, in the case of the negative magnetostrictive coefficient, the action is opposite to the case of the positive magnetostrictive coefficient. Thus, the direction of the magnetization of the magnetic material is changes to align towards a compressive stress or away from a tensile stress.
The inverse-magnetostrictive effect can be used to change the direction of the magnetization of the free layer in the spin-valve film. When an external strain is applied to the spin-valve film, an angular difference between the magnetization of the reference layer and the magnetization of the free layer is generated. This angular difference causes the resistance of the spin-valve film to change through the MR effect. Thus, the spin-valve film can be used as the basis of the strain sensor element.